The concept of coherence was introduced to describe a state in which the concentration velocities of all components present within a composition are equal. A slightly less stringent state termed 'partial coherence' is defined, in which a particular sub-group of components may attain a state of coherence before the complete group of components does. Examples of physical systems in which partial coherence may occur are described: ion exchange with ideal co-ion exclusion, ion-exchange with clays in the presence of surfactant, two-phase flow with sorbable components partitioning between solid and aqueous phases, and precipitating and dissolving solids. The chief advantage of recognising systems as partially coherent is that it makes their analysis more straightforward under conditions previously regarded as non-coherent.